cpp-common/bt2: remove redundant assertions

cpp-common/bt2: remove redundant assertions

Remove some redundant BT_ASSERT and BT_ASSERT_DBG.

 - Most `as*()` methods call a constructor that does the exact same
   check.  Some exceptions are field class types that have signed and
   unsigned variants, where the base class' assertion does not check the
   signedness.  Another one is `CommonNullValue::asNull`, where the
   `CommonNullValue` constructor does not take a lib object pointer, so
   does not do a check.
 - Things that end up constructing a `BorrowedObject` don't need to do
   a null pointer check, as `BorrowedObject`'s constructor does one.
 - `BorrowedObjectIterator::operator*` does not need an assertion on the
   length, as `operator[]` of the container is expected to do it (either
   directly, or rely on the preconditions in the lib).

Change-Id: I6f11680628607eee11164a2e9d3f9de7fc7468cb
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12052
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>